776 GEOLOGY. 



flowers, the fourth with moss and lichens, and the rest being destitute of vegetation. It 

 is not the subsidence of the lake, but the elevation of the land, that is here indicated ; 

 for, owing to the constant additions of detritus made to their bed, the natural tendency of 

 the water of lakes is towards a rise of level. This effect transpires when no counteract- 

 in" 1 agencies operate, and appears in several of the Caledonian lochs. Mr. Stevenson 

 mentions, in the case of Lodh Lomond, the site of a house in the village of Luss as being 

 permanently under the summer water mark, while the gable of another house became in 

 danger of being washed down by the rise of the waters of the loch. 



Lacustrine deposits proceed most rapidly at the points of junction with the in-flowing 

 rivers and streams, of which we have an example in the extensive alluvial formation pro 

 duced since the time of the Romans, in the lake of Geneva, where the Rhone pours its 

 turbid and discoloured waters into it, bringing down a large amount of Alpine debris. 

 At such points the disintegrated material is laid down, and the layer is gradually thick 

 ened and advanced forward by fresh accumulations, aquatic plants growing upon it form 

 ing peat by their annual decay, the shells of fresh-water molluscs and other organic 

 exuviae becoming imbedded in the strata. It is easy to conceive of the entire obliteration 

 of a lake by this process of shoaling up, first being converted into a swamp, and after 

 wards acquiring consistency so as to become a tract of firm cultivable soil, lying along the 

 banks of the river from which it has sprung, or otherwise deserted by it, the parent river 

 having been diverted into another direction. Such, in fact, has been the origin of many 

 of the richest districts which are arable and pasture lands, of which we have the evidence 

 in the presence of lacustrine shells, and the character of the beds in which they are depo 

 sited. Marls of various composition calcareous, siliceous, and aluminous are the 

 most abundant and important formations of lacustrine origin. Calcareous marl is pro 

 duced partly by the deposition of the carbonate of lime from solution in the water, sup- 

 plied to the lakes by means of springs, and partly by the decay of the shells of molluscous 

 animals inhabiting their waters, an abundance of which, either entire or but little broken, 

 likewise enter into it, with traces of clay and earthy matter. On this account it is fre 

 quently called shell-marl, and is of a soft friable nature ; but, when solidified, it receives 

 the title of rock-marl. This product, so valuable in agriculture, may readily be detected 

 when no trace of organic structure presents itself, by testing it with an acid the oil of 

 vitriol, aquafortis, muriatic acid, or strong vinegar when, if the substance effervesce 

 upon the acid being applied, it is proved to be genuine calcareous marl. In the United 

 States, beds occur, several feet thick, and covering many hundred acres ; and, for the 

 agriculturist of a future age, there appears from the following statement to be a suffi 

 cient supply in course of preparation : "At Milk Pond, in New Jersey, countless 

 myriads of bleached shells of the families of Limnceana and Peristomiana, analogous to 

 species now living in the adjoining waters, line and form the shores of the whole circum 

 ference of the lake to the length and depth of many fathoms. Thousands of tons of 

 these small species, in a state of perfect whiteness, might be used for agricultural pur 

 poses. In one case a perforation was made ten or twelve feet deep, and did not pass 

 through the mass. It forms the whole basin of the lake, and may at some future time 

 become a tufaceous lacustrine deposit." 



In the aluminous marl, clay predominates ; and, in the siliceous, silica, with little or no 

 calcareous matter. The siliceous marl has been found to consist almost entirely of the 

 indestructible siliceous shields or frustules of minute microscopic plants, called diatoms, 

 which have lived and died in countless numbers in the pools at the bottom of which this 

 substance has been deposited. The Berg-mehl, or mountain-meal of Germany (resembling 

 flour, and mixed with it in food in seasons of scarcity), is chiefly composed of them ; and 

 likewise the Bog Iron Ore, a paste of fine earthy matter, strongly tinged with peroxide of iron, 



